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How do wings generate lift?
Just the basic question that every aviation enthusiast must be curious about: exactly how does a wing generate lift?

Similar: How do I explain what makes an airplane fly to Grandma?

@Farhan - Similar, but I'd like something detailed here if at all possible.  It would be nice to have a really great answer to this question on our site.

There are a number of questions discussing this concept thoroughly. I'll put a few here which are more related.

@Farhan Feel free, I'm sure I've seen most of them.  I'm more just trying to create a really simple way for people to find the answer to this question on this site.  Hence my asking for a detailed, but hopefully easily read, answer.

Related: Why is the wrong explanation of “air travels a longer distance and creates a lift” so popular?

Related: How would you explain Bernoulli to a nine year old?

Related: What makes an airplane fly?

Also search for the "Kutta Condition".  It all depends how complicated we want this particular answer to be...

Another pertinent link: http://physics.stackexchange.com/q/290/276

Related: Principle of aerodynamic lift: are misconceptions also taught in flight schools?

If someone would just roll all these links into a usable super answer...

Some years back, I saw a writeup, that quoted a NASA guy as saying "We know how to optimize an airfoil.  We DON'T know how it actually works."

@JohnR.Strohm That's amazingly interesting !! I think wing generates lift because it does, our job is to find tools (solving equations) to calculate the lift

This https://www.youtube.com/watch?v=zp1KzGQdouI shows that motion/lift is possible without Bernoulli.

To get to the bottom of it, it might help to look at lift at a molecular level:
Every air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects:

Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise.
Pressure means that air particles oscillate all the time and bounce into other air particles. The more bouncing, the more force they exert on their surroundings.
Viscosity means that air molecules, because of this oscillation, tend to assume the speed and direction of their neighbors.

Flow over the upper side of the wing
Now to the airflow: When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Due to the acceleration, the packet will be stretched lengthwise and its pressure drops in sync with it picking up speed. Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. This contraction is needed to make space for that wing; in supersonic flow it will decelerate for the same purpose. Once there, it will "see" that the wing below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly, the packet will change course and follow the wing's contour. This requires even lower pressure, to make the molecules change their direction. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction.
Note that lift can only happen if the upper contour of the wing will slope downwards and away from the initial path of the air flowing around the wing's leading edge. This could either be camber or angle of attack - both will have the same effect. Since camber allows for a gradual change of the contour, it is more efficient than angle of attack.
Flow over the lower side of the wing
A packet of air which ends up below the wing will experience less uplift and acceleration, and in the convex part of highly cambered airfoils it will experience a compression. It also has to change its flow path, because the cambered and/or inclined wing will push the air below it downwards, creating more pressure and more bouncing from above for our packet below the wing. When both packets arrive at the trailing edge, they will have picked up some downward speed.

Behind the wing, both packets will continue along their downward path for a while due to inertia and push other air below them down and sideways. Above them, this air, having been pushed sideways before, will now fill the space above our two packets. Macroscopically, this looks like two big vortices. But the air in these vortices cannot act on the wing anymore, so it will not affect drag or lift. See here for more on that effect, including pretty pictures.
Lift can be explained in several, equivalent ways
Following the picture of a pressure field outlined above, lift is the difference of pressure between upper and lower surface of the wing. The molecules will bounce against the wing skin more at the lower side than at the upper side, and the difference is lift.
Or you look at the macroscopic picture: A certain mass of air has been accelerated downwards by the wing, and this required a force to act on that air. This force is what keeps the aircraft up in the air: Lift.
If you look at the wing as a black box and only pay attention to the impulse of the inflowing and outflowing air, the wing will change the impulse by adding a downward component. The reaction force of this impulse change is lift.
Either way, you will arrive at the same result. By the way: Most of the directional change happens in the forward part of the airfoil, not at the trailing edge!

Lift is a matter of definition
Lift and induced drag are both part of the pressures acting on the wing. If you add up all the pressure forces acting on a wing, their resulting vector will point slightly backwards. The streamwise component is drag, and the component orthogonal to the direction of movement is lift. This is just a defininion, made for simplicity.

This is excellent, I especially appreciated the mini lecture on molecules, I think that really helped me to understand.  For anyone else reading this, btw, make sure you look at DanHumes answer as well, it goes over some of the common myths about how lift is generated.  It's also very useful.

This was a great answer!

Great answer. This https://www.youtube.com/watch?v=zp1KzGQdouI shows that motion/lift is possible without Bernoulli.

I have a question about flow over the lower side of the wing: is pressure there higher than ambient or just "less reduced" than over the upper side of the wing? And I read somewhere that air over lower side of the wing is slow down and then sped up, is this true? Or the air is just "less accelerated: than over the wing upper surface?

@Konrad The details depend on the thickness of the airfoil. On thin airfoils at high angle of attack, flow over the lower side is slowed down and pressure is higher than ambient. In most cases the pressure and speed are close to ambient. On thick airfoils at low angle of attack your last sentence is correct: Air will be less accelerated on the lower side. At the end of its run, the air will assume ambient speed and pressure again, so it will speed up or slow down depending on the state it had before.

Your explanation begins with a low pressure area at the upper surface of the wing. What created that low pressure is the $50 million question. I believe your statement "The molecules will bounce against the wing skin more at the lower side than at the [retreating] upper side, and the difference is lift." with [explanation] is all that's needed to answer the question. I had seen it in a US Navy aerodynamics text book ~ 45 years ago!

@PeteP. The low pressure is the consequence of air trying to follow a curved and/or inclined contour once the wing starts to move. If it wouldn't, movement would leave a vacuum, and you know what Nature abhors most. Now where  are my $50 million?

@PeterKämpf Can I think of the air acceleration as "flow entrainment"? This is due to the relative speed difference (therefore pressure difference) between two bodies of fluid, in this case of an airfoil, the speed difference is between air trying to follow an inclined contour (accelerated downward), and air in the front (NOT accelerated downward), so that air is sucked (entrained) into the low pressure region created by downward flowing air.

@Frank I guess you can, but nobody else does (to my knowledge).

When it comes to the Newtonian explanation, everyone always focuses on the air exerting an upward force on the wing. However the wing is also redirecting some air to go above it which should cause that air to exert a downward force on the wing in response. Why doesn't this cancel out the upward force from the flow under the wing? I'm yet to see this addressed on explanations I see online.

@Paghillect Newton's theory of lift is wrong. If you look at the forces on a wing, the air on top has less pressure, so less force per area than the air below. I wonder what you are missing.

The molecules bouncing off the bottom exert more force on the wing (Lift is force, a change in momentum, not pressure!), than the ones bouncing off the top because of the impact angle, which is due to the positive angle of attack. Push forward on the stick enough to make the AOA negative and the Lift goes negative too.

All of the other explanations confuse causation with correlation.  Nothing that occurs later in time can be credited as the cause of something that occurred earlier.

Short answer: by exerting a downward force on the air around them.
Long answer: Some outreach people at NASA's Glenn Research Center have written up a very good multi-page explanation, dealing individually with each contributing effect, as well as some discussion of why explanations you might have heard at school don't work. Since the navigation there is a bit funky, I'll link each page individually with a short summary.
Lift from Pressure-Area
When a fluid moves over an object (or vice-versa), the pressure is different at different points. Because of this pressure difference, there's an overall force. You can use the Bernoulli equation to work out this force, but you need to know the speed of the fluid (at each point on the wing) to start. You can't just explain it with "the Bernoulli effect", because the Bernoulli effect applies just as much to anything moving through the air.
Lift from Flow Turning
Both surfaces of the wing turn the flow of air. The bottom surface deflects it (the air bounces off the wing), while the curved top surface bends it around (the air sticks to the wing). The turning of the flow is what gives you lift rather than just drag. You can look at the turning as the source of the pressure difference in the Bernoulli effect, or you can think of it simply in terms of equal and opposite forces.
There's another way of modelling the flow turning, which isn't discussed on the NASA site. If you've heard of the Kutta-Joukowski theorem, this is what it relates to. When the air bends around the wing (or any object), there are two special points. At the front of the wing, some of the air goes over the top, and some goes under the bottom, but there's a point in between the two. The opposite situation happens at the back of the wing, where the air from the top surface meets the air that came the bottom way (but not the 'same' air: see wrong theory #1 below). These two points are called stagnation points. In a normal object, they're at the same level vertically each other, but because the back of a wing is sharp, the rear stagnation point will form behind it when the wing is moving quickly enough. That's lower than the front stagnation point, which implies that the net movement of air is downwards. That's where the flow turning comes from, and the theorem lets you calculate how much lift you get.
Wrong Theory #1: Equal transit time
As I said, to invoke the Bernoulli effect, you have to explain why the air on the upper surface is moving faster. Teachers often claim that it's because the air on the top surface has to meet the air on the bottom surface. That's simply wrong, and there's a nice simulator to demonstrate it.
Wrong Theory #2: Skipping stone
This page discusses when people realise the air "bounces off" the bottom surface of the wing, but neglect the top surface.
Wrong Theory #3: Venturi
Some people imagine the top surface of the wing as a half of a Venturi nozzle (a nozzle which speeds up fluid flow by constricting it). This speed difference would give rise to a pressure difference (Bernoulli effect again), but it turns out the wing doesn't work like a nozzle at all.
Bernoulli and Newton
This last page just sums up that the wrong theories start with well-known physics (Newton's laws or the Bernoulli effect), but then try to oversimplify everything to make them fit the situation, so they end up with explanations which make wrong predictions.

In my opinion the easiest to grasp is the flow turning explanation. I mean, you can almost feel it ;]

-1 for wrong explanation of Kutta-Joukowski theorem and flow turning. One should remember that flow turning is the effect of the lift (Which was created by pressure difference), rather than the cause of lift.

@VictorJuliet: Neither is cause and effect. They are both properties of the fluid flow. However for explanation purposes, the direction in this answer is correct, mainly because the opposite direction is not possible; you can derive lift from Kutta-Joukowski theorem, but you can't derive Kutta-Joukowski theorem from lift.

The only wrong thing about explanation of the Kutta-Joukowski theorem is that it does not mention reason why the rear stagnation point moves, which is inertia of the air.

Your use of the joukowski's theorem is to prove that the rear stagnation point is below the front stagnation point. Well, the rear stagnation point moves below the front one because of the Angle of Attack, not because of the joukowski's theorem, though you could use the theorem to verify that the stagnation points are located at the leading edge and the trailing edge.

@VictorJuliet: I don't see the text to try to prove that the rear stagnation point moves using Kutta-Joukowski's theorem (which just states that it does and how to derive lift from it). It does not explain it. It explains neither why it moves to the trailing edge (inertia of the flow), nor why it moves below the front one (angle of attack + already knowing it is on the trailing edge).

My bad. Had to read again. Sorry.
Though, your point about flow turning, flow turning is an effect of lift generation, not the cause.

@VictorJuliet If it were simply a consequence of the angle of attack, then a wing with an elliptical cross-section would also have a low rear stagnation point, but in fact it doesn't. The sharp trailing edge sets up the Kutta condition. I'm not trying to give a full explanation in one paragraph, just to summarise each link.

Thats what my comment said. The sharp point is the stagnation point (Joukowski's theorem), but it is below the front stagnation point (angle of attack)

The elliptical wing also has a positive angle of attack, but doesn't lift or have a low stagnation point. The Kutta condition is what makes the difference.

@VictorJuliet: Angle of attack can only explain why the aft stagnation point gets below the front one, but before you can do that, you need to establish that it sticks to the trailing edge, which is what the Kutta-Joukowski's theorem is about and the explanation involves inertia (the inertial force overcomes the pressure and prevents the air from flowing around the sharp edge).

@DanHulme Actually the Bernoulli has been proven as not accurate for callculating the lifting force in the air fir airplanes, as the bernoulli equation is only accurate for laminar air flow and not turbulent air flow. As the wings generate turbulent air flow the result you get using his equations do not equal the force you observce when testing a wing in a wind canal for example. Physics student here, had this in my first year of study :)

@RononDex If you'd read the answer, you would have seen that it repeatedly debunks the idea that Bernoulli's principles can completely explain lift, no matter what model you put it in...  So, yes, you are correct.

This https://www.youtube.com/watch?v=zp1KzGQdouI shows that motion/lift is possible without Bernoulli.

HOW AN AIRPLANE GENERATES LIFT
There are usually two popular fields of thought (excluding the debunked equal time theory) behind why an airplane flies; some think it is caused by an application of Newton's 3rd law, and others think it is caused by a pressure difference on the top and bottom of the wing. Basically both the "Newtonian" explanation and the "High/Low Pressure" explanation are right to a certain extent.   NASA acknowledges this (see second reference below) in their article however their ultimate explanation is much more focused on mathematical application and less on a physical explanation.
Newton's 3rd Law

On the Newton's 3rd law side the net aerodynamic force is caused by a redirection of the relative wind downwards (known as "downwash").   If you look at the vector diagram describing the forces by the wing on the air its is shown that this redirection is caused by a force on the wind by the wing which points downwards and more or less perpendicular to the chord line of the wing (the line directly between the leading edge and the trailing edge).   Because of Newton's 3rd law, this results in a force by the wind on the wing in the opposite direction (upwards and more or less perpendicular to the chord line); this upwards net aerodynamic force accounts for lift and induced drag (drag caused by the lifting processes of the airfoil, not to be confused with parasitic drag which is drag caused by the surfaces of the plane; a parachute trailing behind the plane would contribute to parasitic drag, and all airfoils produce some amount of induced drag when they generate lift).
On the bottom of the wing this redirection of air can be explained simply.   The relative wind hits the bottom and is forced away from the airfoil by the airfoil's normal force.
On the top of the wing the air is redirected by a phenomenon known as the Coanda effect, resulting in a laminar flow (the relative wind follows the wing and is directed downwards by it).   I will describe why the wind follows this laminar flow in greater detail when I explain the second major lift generating phenomenon that has to do with pressures (as you will need the information from that section to understand the Coanda effect)
High/Low Pressure

There is a higher air pressure on the bottom of the wing relative to Patm (atmospheric pressure).   This is because airstreams are concentrated when their paths are blocked and redirected by the airfoil.   Higher concentration of air leads to higher pressure.
Likewise on the top of the airfoil airstreams are prevented from directly reaching the top surface of the wing, creating a void where there is a lower concentration of air particles and thus lower pressure.   Because fluids naturally flow from high to low pressure the air at Patm well above the wing is "sucked" downwards and hugs the surface of the wing.   However even with this laminar flow (as we discussed above) there still exists a low pressure zone on the top of the wing; the air from the laminar flow still isn't enough to restore that region to Patm.   This can be found by looking at a pressure map of an airfoil -- you will see that there is a low pressure region on top of the wing even if laminar flow exists.   This section should have also answered why laminar flow exists (see the last part of the newton's 3rd law part above).
Finally, because you have a higher pressure (force per unit of area) on the bottom of the wing than you do on the top of the wing, the forces on the airfoil are unbalanced and point upwards, in a similar direction to the net aerodynamic force caused by newton's third law (detailed above).   This contributes to the net aerodynamic force.
Because of the lower pressure on the top of the wing relative to the bottom, the airflow on the top of the wing moves faster than on the bottom, according to Bernoulli's equation (basically in an airstream a decrease in pressure results in an increase in speed and vice versa) -- See the flow diagram at the top of this post.   This may be why the "equal time" theory (that the airflow on the top of the wing has more distance to travel so it has to travel faster) is so widely accepted.   The airflow on the top does travel faster but not because it's a longer distance.
This also accounts for "wingtip vortices" -- those swirling vortices of air that can be seen (under certain conditions) trailing behind the wings of a plane.   This is because the high pressure air from the bottom of the wing swirls over the ends of the wing to try and neutralize the low pressure area on top (because fluids tend to travel from high to low pressure).   They do increase the pressure on top of the wing (and as a result decrease the pressure on the bottom) somewhat, reducing the pressure difference, however since the airplane is moving not all the air traveling from bottom to top reaches its destination as the airfoil moves out of the way, leaving that air to swirl in a circular vortex.   This stream of high pressure air reduces lift (because it decreases the pressure difference).   This is why winglets were invented (The vertical wing extensions on the end of wings) -- to block some of this flow and increase lift (and therefore fuel efficiency).   "Ground effect", or the phenomenon which increases lift when a plane is close to the ground is due to the ground getting in the way of the air trying to swirl up and neutralize the low pressure on top of the wing.
Final Comments
One more aerodynamic phenomenon that I will relate to this explanation is a "stall".   When an airfoil stalls it looses a large amount of lift and can no longer counteract gravity, causing the plane to plummet to the ground.   As a pilot I have practiced stalls many times and there are two noticeable things that happen leading up to a stall.   One is that the airplane looses airspeed considerably as you start to increase the angle of attack.   In this case what is happening is the total force on the wing is being angled backwards so it is mostly induced drag rather than lift (to a certain point increasing the angle of attack increases lift because it increases the total force on the airfoil however as the angle gets extreme lift starts to decrease and drag continues to increase).   Finally when the airplane stalls you feel a sudden jerk downwards by the airplane as if a cord holding it up were just cut.   In this case the wing has reached its critical angle of attack and the laminar flow on the top of the wing (as detailed above) has separated (because the lower pressure on the top of the wing can no longer pull the wind down to conform with its surface as the necessary force to change the wind's velocity vector by that large angle cannot be exerted by that pressure difference.    Once the airplane stalls you must reattach the laminar flow to the airflow to "recover" from the stall -- in a plane you do this by pitching down with the yoke.

In the future I would love to expand this post with more mathematical explanations on how to calculate the lift of a given airfoil as well as exploring other related stuff like coefficient of lift, Reynolds number, how to calculate critical angle of attack, and related subjects.   This field is generally dominated by empirical data and to break into some of it with some complicated math is hard but fun to do (not to mention the way of the future, especially as computers can now process these mathematical models for us and are much faster at doing so than experiments can be).

Useful sources:

https://web.archive.org/web/20180530165802/http://www.allstar.fiu.edu/aero/airflylvl3.htm

https://grc.nasa.gov/www/k-12/airplane/bernnew.html

https://grc.nasa.gov/www/k-12/airplane/wrong1.html

https://grc.nasa.gov/www/k-12/airplane/wrong2.html

https://grc.nasa.gov/www/k-12/airplane/wrong3.html

+1 for the animated gif, very cool.

This https://www.youtube.com/watch?v=zp1KzGQdouI shows that motion/lift is possible without Bernoulli.

The simplest answer that I know that is that is still accurate is that for any object to move through the air, some force must push the air in front of it out of the way (gravity, engines, momentum etc doesn't matter). If more of the air is pushed downwards then upwards (by for example, wings) then the difference is called lift.

I have to admit, that's a pretty clean high level explanation.

This describes when there is a lift. It does not say anything about why wings, in particular, generate it.

Extend the logic and you will see that there is nothing special about wings. Any shape can generate lift if the circumstances are right, the shapes of wings just happen to be better at pushing more air down then up then, for example, a brick.

How do supersonic airfoils produce lift then?

@Koyovis - the speed of sound through a medium has nothing to do with the lift generated by pushing the medium out of the way. Exactly the same physics apply to an aerofoil through water such as those used in the Americas Cup racing. link

At supersonic speeds, how does the airfoil push air out of the way that is in front of the wing?

@Koyovis I am not getting your question. Speed has nothing to do with it. A force (f=ma) is required to move the medium out of the way, that force comes from the motive power of the vehicle (engines, gravity etc.). The material moved forward pushes back (drag) and the material pushed down pushes up (lift).

Speed has everything to do with it.  No air molecule can exert a force on another that is "upstream" of it in a flow that is moving faster than the speed of that molecule. The molecules in a subsonic flow are still moving in upstream direction. In supersonic flow all the molecules are flowing downstream.

@Paul Smith, the physics of lift in supersonic flow are different than in subsonic flow. There are shock waves involved on the leading and trailing edges on both the top and bottom of an airfoil in supersonic flow, and the pressure changes as air passes through these shocks.

@CharlesBretana - For a brick, a wing, a bullet or an airfoil to move through the air (or any other medium), that medium must be pushed out of the way. Newtons 3rd law means it pushes back and we call the component of that pushback that is opposite to gravity 'lift'.

@CharlesBretana how quickly sound can propagate through the medium does not change the fact that the medium is moved out of the way and doing so requires force. Lift is a component of the medium's reaction to that force.

@Paul, What you fail to comprehend is that the fastest any force can propagate through any medium is at the speed that the individual particles in that medium are moving, and interacting with one another. And the speed of sound in the medium is exactly that, the average speed of the particles (be they atoms o molecules)

And the "push-Back* is not LIFT, it is just the force that the medium exerts on the object. For every force there is an equal and opposite force. This is reflected by the conservation of momentum. Which is NOT a cause of anything, but simply a necessary consequence of the basic laws of motion. The cause is simply the repellant electromagnetic force between the electrons in the outer shells of the particles involved. The push back is a consequence (a result), not a cause, of the force

@CharlesBretana - I think you are confusing different concepts. The force applied to a particle of medium to move it out of the way is independent of how that medium internally propagates that force. The pushback is the consequence of moving the medium, not a property of the medium. That part of the pushback which is normal to gravity IS lift.

@Paul, No I am not confusing anything. When a body moves through a fluid faster than the speed of the fluid particles (Sound, is, by definition, the movement of the fluid particles within the fluid, so this is the speed of sound in that fluid).  Particles can only exert a significant electromagnetic force on other particles when they get close to them (the magnitude is inversely proportional to the square of the distance between them).  So when a body is moving through a medium faster than this speed, the particles can never exert a force on other particles that are upstream from them.

All forces you are talking about are aggregate forces, not basic forces, and are caused by the basic electromagnetic force. But that is an inverse square law force, that falls off rapidly as distance increases.  On top of that, it is repellant  only when the particles generate opposite electric fields. Almost everything in nature is neutral, so the only time the EM force works is when two particles (atoms, molecules), are really close together. When they are even a bit further away, each of them looks more or less neutral to the other and there is no longer an EM force between them.

This principle can be illustrated with a thought experiment. If you could manufacture a steel beam that reached to the moon, and you exerted an enormous force on the end of it at the earth's surface, how long do you think it would be before the other end (on the moon) moved? The answer is determined by the speed of sound in steel. Your approach would violate causality. In your world the other end would move when? Immediately? If not immediately, then when? can things in your world travel faster than the speed of light? what principle governs your answer?

@CharlesBretana - yours is the theory that lead to it being called "the sound barrier" as if it was an impenetrable force. The fact that Chuck Yeager flew through it in 1947 should give you a clue that there is a problem with your thinking. If your thought experiment was valid, then any attempt to fly supersonic must fail because the air could not be pushed out of the way. And yet, we can fly supersonic. In your experiment, it is not the moon that pushes back, it is Netwon's 3rd law. The push back is the reaction to the force applied, not a property of the material it is applied to.

Wings generate lift pushing air downwards. As a kid I used to stick my hand out of the open car window and tilt it - there is an upward force. A flat plate does this.

So aircraft wings could be flat plates, but unfortunately flat plates create a lot of drag as soon as they create lift since the flow at the upper end detaches immediately (curly spiral in picture above). This effect could be reduced by using a cambered plate instead of a flat plate, reducing vortex on the upper surface:

But the issue remains that as soon as the cambered plate is tilted further, it creates a lot of drag, in the same way as the straight flat plate. A water drop shape is more drag efficient than a flat plate, by keeping the flow attached. And what is a wing cross section other than a cambered plate with a water drop cross section?

It gets a bit confusing and all when we look at accelerating air at the top and lower pressure etc, especially if we want to explain the creation of lift from that. Ultimately the lift is created by accelerating the air downwards, and continuity of mass implies that the air on the top side must accelerate. It is an effect rather than a cause.

The flat plate is most efficient at its design angle of attack. Making the airfoil thicker increases drag, but widens the angle of attack range in which it works well.

@PeterKämpf Got you, changed it.

Here is a link to John S. Denker's web book on airfoils. This is probably the definitive explanation of how wings work.  John Denker has a bunch of websites worth checking out.
http://www.av8n.com/how/htm/airfoils.html
Bottom line: for a 150,000 lb. aircraft to stay in the air, it must impart 150,000 lbft of momentum to the air through which it passes. You can talk about air pressure differences (etc.) but that's only the beginning of the explanation.  If you think equal transit time, or wing curvature is what makes wings work, this is a must-read.

Was literally in the middle of reading that link when you posted it.  It is a great read, I agree :).

A simple way to understand it is that the wing acts as a blade in a fan. Moving through the air at the correct angle causes a vacuum to form on top. The front tip must be round to allow the air to move smoothly and expand to create the vacuum.
Flat bottoms and other shapes are simply maximizing this effect but are not necessary. This is why it is possible to fly upside-down as long as the wing is hitting the air at the right angle. (Not at a right-angle.)

lower pressure, yes, but suggesting that a "vacuum" forms is quite wrong.

@Federico Well not a real vacuum. I guess I should say a relative vacuum.

The front tip doesn't actually need to be round to generate lift. BillOer's link explains why. If it were that way, paper planes, kites, and some kinds of gliders wouldn't fly.

@DanHulme I didn't either say it is an ingredient of lift but rather that it is necessary to avoid erratic airflow.

Update: See Own Experiments on Flow turning at the bottom of this post
I'm an independent science journalist, I did a lot of research about myths and false explanations around lift and this explanation is the outcome:
The Problem.
As we know, the principle of the generation of lift in general and the Magnus effect is wrongly understood and explained false in many sources. The high flow speed around an airfoil bulge (or a spinning sphere /cylinder in the case of the Magnus effect) and the related low pressure (Bernoulli effect) is not the cause of the lift as often stated but is just assisting lift generation because it is an acceleration of the air. However, it is still an important factor in the mechanism of lift because it is part of the lift force (Force = Mass x Acceleration). This extra acceleration due to increased flow speed can be added to the normal acceleration that is involved with the force that causes a flow to turn.
The Real Cause.
Also generally accepted is that the real cause of the lift is the air that is turned downwards by the angle or shape of the airfoil and this force causes a force in the opposite direction, as explained by, among others, NASA. Yet, the mechanism is still unclear for many people. I try to give a little more insight with some very easy self-developed experiments and examples that are easy to understand.
(see also this video demonstration). We know that in order to turn a flow, a force is required, so the bigger the deflection, the bigger the force. A turning is actually an acceleration. During the turning there must be an equal force in the opposite direction (Newton’s third law). This is the actual lift on the airfoil. It is clear that a certain radius of flow turning (action) results in an equal radius of the opposite force (reaction). It is important to understand that the reaction of the airfoil on the accelerated airflow is caused by the interaction of the airfoil surface with the boundary layer.
Center of Pressure.
The key to create action=reaction on the airfoil is the viscosity of the air as without the air sticking less or more to the airfoil, the necessary interaction would not happen.These forces act everywhere on the airfoil but the center of pressure (CP) occurs there were the average deflection is the biggest, so there is also the biggest action=reaction point. This is the point were the lift force acts on the airfoil. We can check this easily with deployed flaps. The flaps cause a bigger deflection of the air at the trailing edge, thus the center of pressure moves more to the trailing edge then without flaps.
The Real Lift Force.
As the air is deflected downwards, the air exerts a force in the opposite direction which means that it adds up to the pressure on the underside of the wing with the result a bigger vector in the upward direction. But on the upper side of the wing now we have a smaller vector as the pressure is lowered because here is a deduction of the pressure caused by the force in the upward direction. The result is a net force upwards. This vertical pressure lowering is the real lift force.
Summarizing:
We have a relatively low tangential pressure reduction (acting in the flow direction) which is the Bernoulli part and is the accelerating part of the lift force. And we have a huge vertical pressure reduction which is the Newtonian part of the lift force which actually causes the airfoil to move up and which determines where on the airfoil the center of pressure is located and where the resulting lift force acts. Most of the pressure we see on an isobars figure of an airfoil is vertical and only little is tangential. This corresponds to earlier measurements by aerodynamicists that the pressure reduction in the flow direction (Bernoulli) does not correspond to the actual generated lift. To understand the relation between the pressure reduction in the flow direction and the pressure reduction in the vertical direction, realize that the deflection of a flow in order to create lift is always accompanied by a pressure gradient, so if the flow speeds up over the top of the airfoil and lowers in pressure (Bernoulli’s principle) and then is turned downwards to create an upforce, the flow is decelerated and the pressure increases. This increase of pressure on the upper side of the airfoil is negligible compared to the decrease in pressure on the upper side caused by the air that is accelerated downwards, hence the airfoil moves up and we have lift.
One more Example.
Imagine a flat plate wing flying at zero angle of attack with at the trailing edge a flap that is pointed downwards. Imagine only the airflow on the upper side of this wing. There is no acceleration and related pressure lowering of the flow as the flow doesn't pass any obstacle. It just encounters an adverse pressure gradient when it moves over the flap down because there is a decrease in flow speed thus an increase in flow pressure (Bernoulli). But as the flow is deflected down, a force in opposite direction acts at the same time and therefore on the upper side there is a much, much more important pressure decrease (because the force in the upward direction works against the ambient pressure coming from above). This decrease in pressure caused by the 'vertical' action is the real lift force.
Update: Own Experiments on Flow Turning. On september 26, 2018, during personal flow turning test experiments with self developed cardboard flying wing devices, I strongly found evidence for a theory that I had long suspected. This involves the importance of the distance of flow turning in relation to the steepness of the turning. Explained briefly: The distance of turning seems more important then the angle of turning. When throwing the wing, and when estimating the location of the center of pressure, the side with the longest turning always won it from the side with the steepest turning, no matter what the orientation of the wing was.
The test results:
--Short steep curve pointing downwards in the front, long less steep curve in the back pointing upwards.> Result: positive momentum, nose moves up.This is the effect of the curve in the back as a predominant down pointing curve in the front would generate a nose-down moment as this would be a negative angle of attack.
--Long less steep curve pointing upwards in the front, short steep curve in the back pointing downwards.> Result: positive momentum, nose moves up. This is the effect of the long less steep curve in the front as this is a positive angle of attack.
The results of my findings correspond to the fact that the flow turning at the leading edge of an airfoil is acually the biggest while it is not creating the biggest momentum. The turning to the trailing edge after the point of maximum camber however is longer, it wins, so it creates the CP momentum. It seems logical however that in a battle between two curves of the same length, the curve with the steepest angle wins.
One of my self-developed devices to do experiments with lift, flow turning and center of pressure: The FWSCLm Demonstrator (Flying Wing Stability & CL movement). The pen in the front can be moved in and out in order to regulate the center of gravity. The flaps in the back are used to increase or decrease the curvature of the wing profile in order to regulate the center of lift.
 side-view

Really hear you about the down votes with no comments, but if you stick with it much to be learned on this site.  Your wing indeed looks like slow, high lifters found at Airfoil Tools on the net.  I have also found that thin under cambered wings make for delightfully slow (walking speed) balsa gliders.  You may find thinner wings are better for wind penetration (less drag). Comparisons of eagle and albatross wings can give good insights on wing design.

Thanks for your comments about the wing. The thick wing high lift profile was disigned for particular tests on flow turning to see a strengthened effect in short flight. As you said, thinner is better for less drag. I also have a curved flat plate version of this wing with flexible curve. Here you see it in action. The video actually shows the auto pitch correction: https://vimeo.com/303720825?ref=fb-share&1&fbclid=IwAR2U1S1uyCpKwAKYQEJe-UOb-JDa9RZf1bLzG7PJbBdden0b6EUyEg9tuQc

Recommend also studying sails, particularly the jib sail.  "Acceleration" of air across the top of the wing is nonsense.  Nor is air a "fluid", it is a compressible gas.  Flow turning indeed is related to low pressure on top of the wing. The great Coanda realized the deflected air flow creates a local low that the wing (up) and the airstream (down) tries to fill.

However, we cannot forget "viscous" effect of moving air (or water) pulling surrounding air into the stream. A simple sink top aspirator creates a strong vacuum. Lift force is also created by airstream striking an angled surface (bottom of wing).  There is more than one source of lift.  I continue to try to understand which one is most EFFICIENT.  It may be the lift over the top of the airfoil, as covering the bottom of the wing seems to make my gliders go faster and farther.

And finally air ram effects (higher pressure) underneath the wing present in undercambered wings (note "capped" wing tips of U2) and parachutes.  I believe this to be most draggy and inefficient, but can make for very slow flying speeds!

Acceleration  over the top of the wing is indeed nonsense as you said in your second comment. I always take for this an example a flat plate with a flap at the trailing edge, such as one of my cardboard flyers. If the plate is flying with the leading edge at zero AoA, there is no acceleration of air around the top, rather a deceleration due to the adverse pressure gradient above the flap. Still we have lift because of the flow turning by the flap. It is all about forces. Turning af air means acceleration of air.

And an acceleration doesn't always mean an increase in speed such as is the case with acceleration in the direction of movement. In the case of flow turning it is the force that is required to turn the air molecules into an other direction.

Agree there, turning at constant speed is acceleration.  With your video, the upraised trailing edge deflection force over rides the more efficient Coanda over the top.  It is in effect a very large aileron.  However, a smaller (reflexed) upward trailing edge is used on flying wings.  I go with birds though.  Let the wing be the wing, and keep H stab for pitch control.  Good video though!

Both wings, the version with profile and the curved flat wing, have a pen that can be moved in and out to change the CG. The trailing edge flaps are set to adjust the trailing edge to the CG position, as a function of horizontal stabilizer. On the flat wing, the decalage is amazingly low with a sufficient forward (stable) CG position. It almost looks like a flat plate, so drag and trim drag are very low!

This is a very instructive article published in 'The Physics Teacher' (Vol. 36. Nov. 1998). It includes mathematical derivations and a numerical example:
https://i.sstatic.net/YRgY5.jpg
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https://i.sstatic.net/OCXt5.jpg
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How does a small ball generate centripetal force when it moves on a curved surface? The reason is gravity. When the small ball has a velocity along the red arrow, the small ball has a tendency to leave along the normal direction of the surface, so the force of the small ball on the curved surface will be reduced, thus the centripetal force of the small ball moving along the surface will be obtained.
We change the small balls on the surface into air. When the air does not move, assume that the force of air on the curved surface is F, and when the air has a velocity along the direction of the red arrow, the force of air on the curved surface is f, because the air has a tendency to leave along the normal direction of the curved surface, so F > f. So air has a centripetal force moving along a curved surface, which makes air move along a curved surface.
The force exerted by air on the curved surface is air pressure. A decrease in air pressure is a decrease in the force exerted by air on a curved surface.
The curved surface here is similar to the wing.

I don't agree to this answer. The mentioning of gravity only makes matters difficult, as people can think gravity is involved in the creation of lift. A better image would have the ball travelling on a straight line, and colliding with the curved surface. This avoids the need for gravity, and makes the analogy with an airfoil better.
Furthermore, if there is no curvature, the pressure also decreases, which doesn't show from your explanation.

@ROIMaison Note that for air I'm talking about the normal movement trend, which leads to a decrease in pressure.

@ROIMaison https://aviation.stackexchange.com/a/70283/42162

Lift is a force generated across a wing because of Pressure Difference. So,basically If you are able to achieve different pressure above and below a wing, you'd have lift. Now, from basic Newton's law, this force would be directed from the region of high pressure to the region of low pressure (Because the region of high pressure is going to push the surface by exerting more force onto it as compared to the region of low pressure which would push the surface with a relatively lesser force).
Now, the important thing is to create this pressure difference. This is achieved by exploiting an interesting property of fluid : A fast flowing fluid has lower pressure as compared to a slow moving fluid. This property can be proved by various mathematical means and is beautifully incorporated in the Bernoulli's Principle. Hence Bernoulli's Principle is a mathematical expression of an inherent property of a fluid.
Now, to get lift, the required pressure difference can be created by having a flow around the airfoil in such a way that the velocities of fluid below and above the airfoil are different. This is achieved by changing the shape of the wing (Camber) in such a way that it becomes asymmetrical. The asymmetry causes different velocities on the top and bottom portion of the airfoil because of the following reason:
When a fluid reaches the Leading Edge of the airfoil, some part of the fluid is displaced upwards, while some of it is displaced downwards. Due to the asymmetry of the Airfoil, the fluid which has moved upwards has less cross sectional area to move through as compared to the fluid which went under the airfoil. This difference in area available to the fluid for movement creates the difference in the speeds of the fluid in different regions. This property of fluid to move faster in areas of lesser cross section, and move slowly in areas of larger cross section can be derived in mathematical form by application of conservation of mass, and is called Principle of Continuity.
Hence, changed fluid velocities create a pressure gradient which in turn causes a force on the wing, which is called lift. Now, this lift can be in any direction (which could be found out by integrating very small forces on very small areas on the wing surface). The component of this force perpendicular to the direction of the velocity of the aircraft is called lift force, where as the other component parallel to the velocity of the airplane is then included in the drag force.
EDIT
For very accurate representation of the equations governing fluid behavior, it can be argued that Bernoulli's Principle is incorrect. In this case Navier Stoke's equation is valid, but for understanding purposes, any time invariant (steady), in compressible, inviscid flow can be considered to obey Bernoulli's Equation.
Further, for a real fluid, it would not obey Bernoulli's Equation most of the times, but the general behavior of the pressure reduction with flow speed increase is still observed, though exact pressure drop can not be calculated through Bernoulli's Equation. In such cases Navier Stoke's equation is used to correctly calculate the pressure drop created due to increased velocity of the flow.
EDIT 2
For symmetrical wings, the wing wont generate any lift if the flow see's the wing symmetrically, so that inherently means that a symmetrical wing with 0 angle of attact would not produce any lift. To get lift from a symmetrical wing, it is placed at some angle to the flow, so that the flow see's it "asymmetrically" and hence, the above explanation can be used to explain the life generated in this case.
EDIT 3
Explanation for Planes flying upside down: For a normal plane to fly, a positive angle of attack is needed. Give this plane a velocity axis roll of 180 degrees, you get a plane with -ve angle of attack, and hence a negative lift. But a plane cannot sustain flight with negative lift, so what the upside down flying planes need to do is to increase the -ve angle of attack to positive, by pulling the nose up (that would be pushing the nose towards the sky in an upside down plane). This causes the angle of attack to change and become +ve. The +ve angle of attack means that the wing will now experience a life such that a upside down plane has lift in upwards direction (This is equivalent to a normal plane with - ve angle of attack and hence negative lift).

This doesn't explain why a wing with no camber, or one with a symmetric cross-section top-bottom, or one with a longer bottom surface than top surface, can generate lift.

@DanHulme +1 or how planes with camber can fly upside down.

@Jan Hudec , you should understand the difference between Bernoulli's Principle and the equation. The theorem states : "In fluid dynamics, Bernoulli's principle states that for an inviscid flow of a nonconducting fluid, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy."  where as the equation, on the other hand tried to get quantitative results of bernoulli's principle, but fails to get so because of the fact that it predicts wrong results

Bernoulli's principle is qualitatively true, quantitatively not. And hence the difference.

Quantitatively is produces incorrect results because of its inherent approximations

THIS ANSWER IS WRONG. Bernoulli's equation holds to sufficient precision around the wing. But Bernoulli's equation needs velocity to derive pressure and the explanation of why there is a higher velocity above the wing is incorrect. The area above and below the wing is not delimited, so the air has plenty of freedom to choose it's velocity distribution. It also does not match reality, because the area increases above the wing from front to back and decreases below the wing in similar fashion, but the velocity distributions don't follow similar profile.

The answer is incorrect only if you ignore boundary layer effects

Yay, @Victor! All these answers ignore the fact that the only molecules of air that can create any lift are the ones directly in contact, (that actually physically IMPACT) the airfoil surface. And, as we know, these molecules are, at least momentarily, completely stopped with resect to the airfoil.  It is the affect that all the other things that are happening has on the normal velocity of these boundary layer molecules at the moment they impact the airfoil surface, that determines the pressure they exert, and the lift they will generate.

I'm downvoting this answer because it asserts that "a fast flowing fluid has lower pressure as compared to a slow moving fluid". That's a very inaccurate statement. The compressed air inside a pneumatic hose has higher pressure than the slower-moving air surrounding the hose.

A plane flies by several mechanisms. The first is the Bernoulli effect caused by wing camber that generates a pressure differential pushing the wing upwards as it moves forwards through the air. Note that birds have cambered wings. However, it is possible to have a plane with completely flat wings and no camber at all, so it is a mistake to think this is the only source of lift (as some of the answers above have done).
The angle at the wing root is also important. If you stick your hand at an angle out of car window, you will feel it forced upwards. This same effect is accomplished in an aircraft by angling the wings slightly upward relative to the plane of the fuselage.
Finally, you should be aware that the reason a plane stays aloft has nothing to do with lift, but with the surface area it presents to the ground. The primary force holding a plane up is air resistance which is a function of this surface area. The force of this air resistance is much greater than the force generated by the previous two effects. For example, a major design criteria for a plane is whether it has a square fuselage or a round/oval fuselage. A square fuselage will present more surface area to the ground, thus having greater efficiency in staying aloft. For this reason, nearly all early aircraft had square fuselages. However, a round fuselage will be more efficient moving forwards than a square one, so in a plane built for speed, round is better. An aircraft with a round fuselage goes faster, but is less fuel efficient than one with a square fuselage.
The same argument holds true for wing area. The larger the wing, the more air resistance. For this reason, gliders have relatively large wings compared to powered aircraft. The drawback of a large wing is the same as that of a square fuselage: the plane goes slower.
So, to recap, there are three factors that keep an aircraft aloft: vertical air resistance due to downward facing surface area, the angle of the wings at the wing root, and the Bernoulli effect associated with the camber in the wings.

Paragraph 3 makes my head hurt... not that the rest is much better. In the spirit of actually pointing out specific things that can be addressed, try this: for a square and a circle of the same area, the circle will have a larger diameter than the square's side, therefore a circular fuselage of the same internal volume will present more, not less, surface projected on the ground, for all the (little to zero) good that will do to your plane.


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What is the purpose of D-Bag in skydiving parachute?
Recently I had the opportunity to take a closer look at the skydiving parachute. And I could not explain to my self the reason to use D-Bag and not pack main canopy directly in backpack compartment.
I know that the reserve canopy is packed directly into the backpack. And the sequence for deploying reserve canopy is:

Sky diver pulls release cord, which pulls out the pin that holds the backpack flaps close
Spring loaded drogue parachute jumps out into the airstream
Drogue chute pulls out the main canopy which opens
Sky diver does not flatten him or herself on the ground.

More puzzling is the main canopy assembly. As far as I could understand the main canopy is packed into so called D-Bag. This D-Bag is then inserted into container in the main backpack. Why this intermediate step?
The deployment sequence of main canopy is described here: http://adventure.howstuffworks.com/skydiving4.htm
I wonder what happens to the D-Bag after deployment. Does it hang loose? Does it remain close to the canopy on the upper side?
As far as I understand the drogue chute remains attached on the back side of the canopy and acts as a sort of stabilizer. But what about D-Bag? Does it just flap around?
Why not pack main canopy in separate compartment of backpack directly?

Qualifier: I am a master parachute rigger.
The bag serves two functions - it allows the suspension lines to straighten out in a controlled manner, and it prevents the canopy from inflating before the lines are fully stretched out.
It is not always required - people who want a very fast opening can pack without a bag, I have done this many times myself. However, you do NOT want to do this after a drop of more than about 5 seconds. These jumpers open the main canopy immediately after exiting the aircraft.
On main canopies the bag and pilot chute are connected to a ring on the top of the canopy. The assembly just follows behind. Some canopies have a mechanism to collapse the pilot chute and/or retract it onto the top of the canopy.
The bag for the reserve canopy is not connected. This allows the reserve to still deploy if the pilot chute gets caught on something else, the bridle is extra long (longer than the suspension lines). It falls separately and usually lands within a kilometer of the airport, near the main canopy if that one was released when it malfunctioned. As they cost about $200 a set the jumpers will put considerable effort into finding it. We usually don't find it in high crops, and we usually don't try to find it at some places in Florida where there is an alligator ranch beside the airport.

As a master parachute rigger, I'm sure your answer is 100% spot on. As someone who questions the sanity of leaving a perfectly functional aircraft while it's in the air, would you mind updating your answer with a picture that shows all the different bits you're talking about, or include some links to those items? It would make a great answer excellent!

@Freeman This is a good link: http://adventure.howstuffworks.com/skydiving1.htm

@FreeMan aircraft have thousands of moving parts working under pressure, force and temperature extremes. If something goes wrong, you will be a statistic. I will be departing before that happens with my no-moving-parts sack of nylon. Also, check your maintenance book and ADs - are you sure it's "perfectly functional"?

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Would a homemade lawn chair balloon be visible on ATC and collision avoidance radar?
I am not a pilot, but have a question about radar detection of improvised lawn chair balloons that I thought might be on-topic here.

This BBC news article describes a marketing stunt gone awry involving a lawn chair, a large number of multicolored helium balloons, and the owner/operator/parachutist
According to the article, he was charged with "mischief causing danger to life".
A similar stunt by American truck driver Lawrence Richard Walters, nicknamed "Lawnchair Larry" or the "Lawn Chair Pilot"  also resulted in fines and legal action.  In that case, the government dropped the fine for "operating a civil aircraft for which there is not currently in effect an airworthiness certificate" because it was not applicable to that type of aircraft. The article also mentions some other attempts to fly lawn chair balloons, including a fatality from crashing at sea.
My question is whether such an improvised manned balloon would be visible on typical airport radar or the independent radar/collision-avoidance systems operated on board large modern passenger aircraft, and thus avoidable.
A 1970s military research paper on radar supporting the need for secondary transponders in detecting light aircraft also mentions detecting doppler from propellers (p.4) in addition to the primary radar reflection, and I would expect the cross section of a lawn chair to perhaps be within an order of magnitude of that of a propeller.   A 2m x 0.5m x 0.5m sparsely filled aluminum chair frame is also still larger than typical microwave radar wavelengths (1Ghz = 30cm wavelength, most radars are going to be above 1Ghz), so it should reflect the radar but of course is a fairly small object compared to large aircraft and therefore the detection range could be lower.

"homemade lawn chair balloon" -- is there any other kind?

Would a homemade lawn chair balloon be visible on ATC and collision avoidance radar?

It might be visible to ATC, but, at best, only as one of several inconspicuous unidentified red dots.
It wouldn't appear on aircraft collision avoidance displays (TCAS) and would not trigger any TCAS alerts.

Radar cross section
Balloons, and clusters of balloons are mostly air (or similarly insubstantial gas). There is a thin layer of plastic but many plastic materials (e.g. mylar) are transparent to radar, or only weakly reflective.
It is for this reason that aerostats and high altitude balloons carry radar-reflectors when they need to be detected by radar.
Whether the chair itself has a detectable radar cross section depends partly on what materials it is constructed from. I suspect most available lawn chairs are not constructed from solid sheets of metal fixed at right angles but, at best, from a relatively tiny amount of relatively small-gauge metal tubing.
I'd be surprised if any improvised lawn-chair balloon had the same radar cross-section as a glider - and those are allegedly hard to see on primary radar.
Some helium balloons are made from metallised mylar (because it keeps the helium in for longer). These may be much more reflective to radar. The Calgary aviator pictured was said to have used "oversized party balloons" - they don't look like the metallised balloons that are commonplace where I live.
A perfectly conducting metal sphere of 1.15m diameter has a radar cross section of 1 m²
Some other radar cross section values:

Human: 1 m²
  small combat aircraft: 2–3 m²
  Cargo aircraft: up to 100 m²

This suggests that military radar might display a return to an operator. A civilian primary radar might be less likely to display a return to an operator.
Primary radar
So far as I can tell, most civilian ATC and powered aircraft (TCAS etc) don't really make much use of primary radar to detect other airborne vehicles. They rely on transponder returns (i.e. secondary radar)
Even a large metal object like a Boeing 777 can disappear from the view of ATC and other aircraft when it turns off its transponder.
The Guardian newspaper reported

Last month, the Swedish government complained that a Russian military aircraft had been flying near its airspace with its transponders turned off to avoid being spotted by civilian radar, and nearly collided with a passenger jet.

I'd be surprised if an improvised lawn-chair balloon was detectable where a Tu-95 was not.
What ATC see
This is from what is said to be a radar recording of flight 77 (at 4:18) probably at Dulles after controllers were aware of the prior 9/11 attacks.

Flight 77 had turned off its transponder, it is one of the small red dots at the left. Here's a closer look at that

Flight 77 is the red dot about 20% in and up from bottom left. There are other red dots that come and go but this one tracks steadily from left to right. I'd guess an especially alert controller who is not very busy would notice this sort of dot.  Even if the radar display doesn't filter it out,  unannounced slow-moving lawn-chair balloons appearing from the suburbs might not always be noticed (though see James' comment to Peter's answer which suggests they often do notice and act on such targets).
Transponders
Radios and transponders designed for hot-air balloons are available (example) and I've read that

Transponders are mandatory for flights in some types of airspace in many parts of Europe from March 2008 onwards. This includes balloons and other light / recreational aviation and aircraft.

So I suspect that authorities will expect aviators in lighter-than air vehicles will take appropriate measures to be in communication with ATC/other aircraft and perhaps to be visible to secondary radar in airspace where they may come into conflict with other aircraft.
Kevin Walsh
Heres a news report from The Milwaukee Sentinel - Jul 19, 1984

Snopes reports

He had wrapped his tether lines in foil in the hope that they would show up on radar, and, sure enough, he was picked up on the screens of controllers at Boston's Logan airport

So you probably need to take specific measures to make yourself more likely to show up on primary radar.
Conclusion
When attaching balloons to your lawn chair, first scrutinize the intertubes for designs for large improvised radar reflectors you can dangle from your vehicle. Hope that nearby on-duty ATC controllers have their displays set to show primary radar returns and not just secondary radar returns.

Although mylar itself is an insulator, mylar balloons can be metal plated, often causing power outages and sparks when they land on uninsulated electrical power lines.  A conductive metal plated balloon should also be radar reflective, as because of skin effect the interior doesn't matter.   I can't tell which kind of balloons are shown.  see also https://en.wikipedia.org/wiki/California_Balloon_Law

@Paul: good point, I'll update my answer.

Re: cluster balloons and transponders: at least one balloonist has flown with a transponder and ATC radios. Modern transponders are pretty efficient, you could power one on a typical balloon flight with a small battery (it's just a question of making sure the antenna is properly mounted).

The balloons in the picture appear to be latex balloons - mylar (metallized or otherwise) balloons typically have a different shape and visible seams.

Thanks for the ATC pictures and secondary sources.  I wonder if there are any primary sources commenting on any of these crazy flights.   I'm not sure what to make of the military plane fiasco, as it is strategically wise to perhaps be a little dishonest about national defenses and say you couldn't see something when you did, etc...

@Paul: I can't find any ATC or FAA reports but I've added a newspaper report about Kevin Walsh - allegedly visible on radar because he wrapped his tether lines in foil.

Primary radars are sensitive enough to pick up even large birds. A lawnchair ballon would certainly register if the settings allow. To suppress the signals from birds and other non-aircraft objects, a minimum speed is required for primary radars to show a blip to the controller. Since the lawnchair is moving with the wind, it will be filtered out, unless the settings are changed, but then a lot of other signals will show up, too. To identify a lawnchair balloon will be almost impossible - the radar would need to track the ballon and distinguish it from birds due to the different, steady motion. I doubt that there is any such algorithm in regular use today.

Yes, I've had this happen several times, with ATC notifying me of a target at such & such bearing & distance, and then seeing birds, or a hang glider, which I suppose isn't that much different than a lawn chair - just tubing & fabric.

My question is whether such an improvised manned balloon would be visible on typical airport radar or the independent radar/collision-avoidance systems operated on board large modern passenger aircraft, and thus avoidable.

Would it show up as a primary radar target? Yes, it could. Primary radar would however rather see the whole bunch of balloons rather than the chair, so it would create a primary target. Primary radar sometimes picks up flocks of birds, too.
Would it show up on TCAS or FLARM? No, as TCAS and FLARM are based on radio signals being broadcast and received between the devices, either transponder or other TCAS, or in the case of FLARM, other aircraft equipped with FLARM. Neither of these devices pick up airborne objects that do not emit signals.

almost certainly the balloons will show up on a radar as a much smaller target than a flock of birds, if they are detectable at all (unless the balloons are metallic) -- spheres generally have a small radar cross section, and latex balloons are made of a non-conductive material, which neither absorbs nor reflects electromagnetic waves of radar wavelengths well at all.

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How do I locate a list of restrictions that would prevent a pilot from flying in the US?
I have a privacy preserving technology that could conceivably help prevent the Germanwings crash of 2014.  The idea would be there is a long list of issues that would prohibit a pilot from flying encoded on a ID card that anonymizes a pilot's information:
I'm guessing that a list might look like this:

Overwork (no sleep)
Simulator experience, test results
Medical issues (temporary)
Drug tests
Psychological issues

A list of medical providers would link the results together, anonymously, and during the pre-flight check, the ID card would be scanned and rather than a specific issue being highlighted, (depression), the pilot would be asked to rest for a while.  No system would know why the rejection occurred, except for the pilot himself.
This might for a balance of privacy, safety, and accountability without compromising need to know.
That being said, I would appreciate any official, or carrier specific list of reasons a pilot would be rejected from flying.  The closest I've gotten to learning about this is MSFT Flight Simulator and a few hours in a Cessna
Supplemental information would be helpful as well (your thoughts about this solution)
Scope
Per the comments below this question, the scope is limited to US

Welcome to aviation.se! your first version was correct, I'd say (IS there A list). As for the questions itself, I'm afraid we are on the verge of too-broad: there are quite a lot of carriers and countries around, one list for each might be quite too much for the format.

@Federico - Limited scope to US (and maybe UK?), and asked for how to find the information, rather than the list itself.  Let me know if that is constrained enough.

I think so, yes. We'll see also what the rest of the community thinks.

I would still split this in to two questions: one for the US and one for the UK at a bare minimum.  It is very (VERY) unlikely to find an expert who can provide one answer to both, and then you will be left with the situation of having two answers, neither of which fully answers the question...

@Lnafziger - will do

The list is exactly one item long:
Any condition which would impair your ability to conduct the flight safely, and in accordance with FAA Regulations.
Any more specific list will miss things and be riddled with loopholes - at some point we have to trust that pilots are sane and responsible individuals, just like we do when we let people get in a car.

From a medical perspective the general medical certification information page at faa.gov is a good starting point, and the FAA's guidance for Aviation Medical Examiners has lots of details.
There are also some disqualifying conditions which are explicitly listed in the regulations, and a list of (some of the) medications which will prevent the FAA from issuing a medical certificate.

The things I noted above will prevent you from getting a medical certificate.  Once you HAVE that certificate the "I'M SAFE" checklist or something equivalent expands upon that and tries to help pilots determine whether they're fit to fly right at this moment. There are various ways of interpreting the acronym, but a common one is:
I for Illness
Do I have an illness or any symptoms of an illness?
M for Medication
Have I been taking prescription or over-the-counter drugs?
If so, do they have a negative affect on my ability to perform this flight?
Are any of them disallowed by the FAA?
S for Stress
Am I under psychological pressure from the job or personal issues?
A for Alcohol
Have I been drinking within eight hours? Within 24 hours?
F for Fatigue
Am I tired and not adequately rested?
E for Eating
Am I adequately nourished, or at least "not hungry"?

There are four very general reasons - really, categories of reasons - that I can think of to ground a pilot (or for a pilot to ground himself):

It would be illegal to fly for obvious procedural reasons, e.g. your license has expired, you've exceeded legal limits on flying hours
It would be illegal to fly for obvious medical reasons, e.g. you're taking a medication that the FAA doesn't allow
You're legal to fly, but your employer's policies don't allow you to fly, e.g. you haven't had some specific internal sign-off required for a certain aircraft, route or crew combination
You shouldn't fly for non-obvious reasons that are known only to yourself, e.g. you're suffering from stress or grief, or have an undisclosed medical condition

But the thing is, airlines already track as much of this information as possible anyway. If they didn't track them, they couldn't schedule pilots on flights efficiently and they have to make sure that every pilot is up to date with medicals, licenses etc. to avoid scheduling problems and lost revenue.
So the only real issue is when a pilot knows he should ground himself but chooses not to. There's no solution to that except to encourage pilots to identify those situations properly, and to have laws and policies that don't penalize pilots who self-ground for good reasons. Even if Germany - or any other country - starts allowing doctors to share private medical data with aviation regulators and airlines, it won't prevent cases where pilots deliberately conceal medical issues.
And you also have to consider that airlines would quickly become very unhappy with any system that says "this pilot can't fly today but we can't tell you why and we can't tell you when he will be able to fly again".
So - at least in my opinion - your proposal mostly reproduces what airlines are already doing, and creates an additional problem of unpredictable staffing. Considering how rare accidents are, it's doubtful if any 'solution' is actually necessary, although I do think it's great that someone is thinking about how to balance privacy with safety.

Perhaps this solution could enhance the systems already in place with more privacy... encouraging those who are fearful of information being disclosed to get help when needed.
Where can I get information on those existing systems?

AOPA has a comprehensive list that answers your question from the medical perspective. The linked resource for each broad category explains the particulars.

Health Conditions That May Affect Certification
In this section, find answers to questions about how a medical
condition might impact your flying privileges. Information is
categorized by physiology and includes the relevant medical standards,
as well as the procedures to follow for recertification, or to obtain
a special issuance medical certificate.
Bone and Joint—Includes information on arthritis and musculoskeletal
conditions.
Cancer—Many pilots recover from cancer and regain flying privileges.
Find out more here.
Ear, Nose, Throat, and Equilibrium—Read the medical standards for
hearing, as well as information on cochlear implants, Eustachian
bypass, and motion sickness.
Endocrine System—Includes information on diabetes and thyroid
conditions.
Gastrointestinal—GERD, hepatitis, colitis, Crohn’s disease and more
are covered here.
Heart and Circulatory System—Read how to get recertified after having
heart-related problems.
Immune System—HIV and related conditions are included in this section.
Mental Health—This section covers ADD/ADHD, depression, psychological
evaluation, and substance abuse.
Neurological (Nervous System)—Read about migraine headaches,
cerebrovascular disease, and strokes.
Pulmonary—Read how you can keep your medical certification if you have
asthma, allergies, or another lung-related condition.
Sleep Disorders—Find out what you’ll need to do to get your medical
renewed if you have sleep apnea.
Substance Abuse—Here are the guidelines for certification if you have
a history of alcohol or drug-related problems.
Urology (includes kidney)—Read how to be recertified after having
kidney stones removed, or after a successful kidney transplant.
Vision—Color vision, glaucoma, and LASIK surgery are all covered here.


text:
Why do pilots say "clear of the active" at non-towered airports?
At the non-towered airport I fly at, pilots when clear of the runway always say "X Traffic, Skyhawk N123 clear of the active." Why not say the runway number? At a one-runway airport, I understand that this could be redundant, but wouldn't it be better to get in the habit of saying the runway number so that--should there be more than one runway--it wouldn't be an issue?

Honestly? It's probably because they heard someone else do it, and thought it sounded cool (or otherwise just got it stuck in their head).
You're 100% right - saying the runway name is, if slightly longer, certainly a bit more informative. It's the kind of thing that can help arriving pilots know which runway is in use, keeping later radio clutter ("Um, uhh, Jeffco Traffic, um, say runway in use?") to a minimum for pilots already on frequency. Even a 'one runway airport' still has two runways (09/27, for example), and it's nice to know which one you should set up for when winds are light.
The preferred terminology is more like "Jeffco Traffic, 3AC, clear 29".

Why does a 'one runway airport' still have two runways?

@GeorgeMillo see: http://aviation.stackexchange.com/questions/18953/1467 (and duplicates)


text:
How much does a typical private pilot pay for a non-training flight?
I realize one can expect to pay upwards of $10k to receive their Private Pilot certificate. Understood entirely.
However, my question revolves around the everyday expense of your average Joe pilot who wants to go out to the local airport and take up, let’s say, a Piper Arrow for a nice relaxing local flight around the area (we will say Kansas City Wheeler Downtown in this case). As a hobby. Maybe take a couple friends or family if they chipped in.
How much money could an average Joe pilot look to spend per flight, taking up a small craft, just because?

I don't know if there's any realistic answer to this. Rental costs can be dramatically different based on location (are you in the US?) and aircraft type. Then there's insurance, software updates and all sorts of other costs (potentially). If you know the training cost per hour in your location then just deduct the instructor's time and that's a good start.

It’d be Midwest US.

@jon, a Google search of FBOs in your area should get you to some websites with local rental rates.  This kind of information isn't hard to find.

I’m not a pilot as of right now, so I’m not familiar with exactly how it works. All I have is flight sim experience. I was simply curious from those who actually are pilots what ballpark expense is, typically. I realize fuel is dynamic and location affects rates, but with a single-engine four-seater craft, has it ever gone above $500?

Ballpark, $150 to $180 an hour for a 4 seater.

I see. I’m putting back $$ for flight school in the future, and was considering whether it’s be worth the investment if flying on an average day was a really expensive thing in the first place.

It isn't a cheap hobby.

@quietflyer - Friends and family are able to chip in if they are passengers. A private pilot must pay their pro rate share of the expenses of a flight. They can collect the rest of the cost from the other passengers.

@Jon - Flying being worth the investment is a very subjective thing. Any hobby a man can have can turn expensive. Before flying, I was a semi-serious cyclist. A good road bike can be over 10k dollars. Add 100 dollars for each set of togs. Race entrance fees vary. If you fish, A bass boat will be expensive. Traveling to fly fish even more expensive. Deep sea fishing even more expensive than that. Rebuilding cars, riding motorcycles, scuba diving, etc. can all be expensive. You find ways to cut costs or have your hobby fund itself. In flying, you can do that by becoming a CFI.

@Jon you should add some information inside the question (your location, the type of aircraft you are thinking of,...)

@ManuH updated with specific aircraft and locality for better ref. I’m not from there, but have been through there before and have friends there. We were talking about how neat it’d be to fly places together, and it got my thought process flowing, which spawned this question.

For a lasting aviation hobby that's more than just "trips around the bay", consider gliding of some description. At the top level, it consists of sailing-style racing tasks around pre determined turnpoints. Typical sailplane rental (UK costs) is ~£25/hr + £10-25 launch fees.

It depends on your location. It will vary based on cost for tie-down or hangar space, fuel costs, maintenance costs, etc. In order to compare apples to apples, let’s  boil it down to a wet-Hobbs-rate.
Wet-Hobbs-Rate is an all-in rate (minus tax) for the use of the aircraft to include everything except away-airport landing fees where applicable. That includes fuel, oil, home-airport landing fees, etc. Wet-Hobbs-Rate is charged on the time that the engine is actually running. Although some Hobbs timers run off of the time that the electrical system is on, most run off the oil pressure sensor of the engine.
Here are some real world examples of the Hobbs Rate costs of rental aircraft in my area.
Cessna 172S with G1000 and autopilot - $175/hr wet-Hobbs.
SportCruiser with Dynon and autopilot - $125/hr wet-Hobbs.
Piper Apache - $250-300/hr wet-Hobbs.
Piper Cherokee - $145-155/hr wet-Hobbs.
Piper Comanche - $225/hr wet-Hobbs.
Cessna 150/152 - $100-125/hr wet-Hobbs.
Cessna 172E-N - $125-175/hr wet-Hobbs.
Robinson R22 - $250-300/hr wet-Hobbs.
Beechcraft Duchess - $250-500/hr wet-Hobbs.
Piper Aztec - $400-600/hr wet-Hobbs.

Wet-Tach-Rate is another way to measure aircraft cost. Wet-Tach-Rate is similar to wet-Hobbs-rate. Except that wet-tach-rate is more like an odometer that calculates time by the amount of engine revolutions. The less RPMs at which you run the engine, the slower the clock moves. More RPMs mean more time calculated. Typically, this means that you can cut your costs down significantly. Your calculated time can be around 75% of your actual time running the engine or less with good engine management.
Most of the flying clubs in my area use Tach Rate as the basis of their costs. In order to rent aircraft from them, you have to be a member of the club. Membership ranges from 30 to 100 dollars a month.
Here are some real world examples of the Tach Rate costs of rental aircraft in my area.
Cessna 150/152 - $50-100/hr wet-tach.
Piper Archer - $75-$150/hr wet-tach.
Piper Cherokee - $50-150/hr wet-tach.
Piper Arrow - $125-150/hr wet-tach.
Cessna 172 - $50-150/hr wet-tach.
Cessna 182 - $75-175/hr wet-tach.
Piper Cherokee Six - $75-150/hr wet-tach.
Beechcraft Bonanza - $150-250/hr wet-tach.

One thing to consider when deciding to join a club or not is their overnight policies. Most rental airplanes are run several hours a day, every day. They do not want to have their airplanes sitting on the ground. During slow season you can rent overnight as long as you pay for a minimum number of hours Hobbs-time each day usually starting at three hours each day.
Most clubs have enough downtime for their aircraft that they will allow you to have the their airplanes for multiple days. Their per day hourly minimum can be as low as one or two hours per day. If you time it right, you can be the last person to sign the plane out after hours on day one. Fly the plane the club’s minimum number of hours the next day. Then sign the plane in the following day before working hours. That would conceivably be 40 hours of use with only a one day minimum to meet.
Something else to consider is that you will have to factor in a minimum of 30 minutes of non-flying time each flight for which you will have to pay. That will be engine or electrical time doing tasks like:

Starting your engine
Getting your ATIS and clearance
Taxiing to the runway of use
Run-up of the engine
Taxiing back to the hangar after landing
Shutting down the engine

You will pay less in Tach-time than you will in Hobbs-time because the engine RPMs will be averaging a much slower rate.
As an aircraft owner, your variable hourly operating costs could be similar to the lower end of the club rates. This would include your fuel, oil, maintenance and other costs. In addition to that, You would have fixed costs for the aircraft of:
Hangar space ($200+/month);
Or Tie-downs ($50-100/month);
Inspections ($1000-2000/year);
Major Overhauls ($10k-$20k at any time)

Lastly, insurance for a renter can be anywhere between 100 and 500 dollars a year depending on coverage. And, more for helicopters. Electronic Flight Bag subscriptions range from 100 to 500 dollars a year. You will need at least two hours of flight instruction every two years to meet flight currency requirements. And, you will need an FAA AME flight physical (time varies by age and privileges desired) unless you want to fly under Basic Med Class III reform or Light Sport Aircraft only. A Medical Certificate will cost around 100 dollars.
You can cut costs dramatically by flying gliders and ultralights. The glider club in my area has a 30 dollar per month membership with its lowest cost glider renting for 10 dollars an hour. You will have to research Ultralight costs yourself.

tach rate not applicable for helicopters for obvious reasons... :)

what are the obvious reasons?

@SwissFrank I'd assume lift (and fuel consumption) being mostly controlled by collective (rotor blade angle) rather than the RPM of the rotor.

I can speak to this from personal experience. I fly the Piper Arrow and have rented from various FBO's on both coasts in various economic areas over the years. The average price I pay per hour is $145 an hour wet. All of the Arrows I have flown have been similarly equipped and of a similar vintage. They have all had at least a single axis auto pilot a 430W and are in fairly decent shape.
The cost really depends on your mission. If I'm just flying the local area with some friends its far more likely I (and many others like me) will fly an Archer which holds the same number of people as the Arrow and is about 25Kts slower. The average price I have paid for an Archer over the years is $119 an hour wet. All of the FBO's I have ever rented from have had both Archers and Arrows (or Warriors) available and if you are checked out in the Arrow most places will let you fly the Archer (slower and non retractable) without an extra checkout.
The Arrow tends to be a "go places" airplane so the flights are a bit longer which will cost you more on a day to day basis but less on a per mile basis (the Arrow is more efficient).
My average \$100 hamburger mission (hey that's me) ends up costing about $225 when all is said and done. If you want to keep it right at \$100 see my answer here. There are lots of great little trips from my home base that are under an hour but with taxi, IFR clearance if I need it, routing, a denied bravo clearance, the scenic way home, or just letting my buddy take the controls for a bit it ends up taking just about an hour of Hobbs time for each leg.

How cool is that? Did you notice we even have a question about the $100 hamburger?

@PerlDuck yes I did notice that, did you notice I had answered that one as well?

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text:
When do I have to contact Centre when departing/arriving at an airport with an ATF?
I'm looking at St-Frédéric airport in Quebec, Canada (CSZ4) and specifically at the "COMM" section. It says that there is an Aerodrome Traffic Frequency (ATF) on 122.8 but it also seems to say that you need to contact Montreal Centre (135.025) on arrival and departure:

Does this mean that Montreal Centre is to be contacted only when the ATF (UNICOM) is not operating, or do I have to contact it at all times?

Since rules vary by country and the question referenced Montreal, I added the Canada tag.

Can you link to or quote the text you're looking at?

I've added a screenshot from the Canada Flight Supplement.

I'm not familiar with the CFS, but what are you seeing that makes you think it might be mandatory to contact APP/DEP? In the US, the frequencies are shown just in case you need them.

I'd have to check other ATF aerodromes to see if they also list centre frequencies but I can't say I've seen this before. Maybe it has something to do with the proximity of this aerodrome to Quebec's class C airspace.

I checked other aerodromes in the area (e.g. CYSG) and they don't list Montreal Centre in the comm section. This seems to be specific to St-Frédéric.

The Aerodrome Traffic Frequency is a frequency for aircraft to use to aid self-sequencing at airports where a control tower is not operational. Another name for it is the Common Traffic Advisory Frequency. 122.8 MHz was the original frequency designated for UNICOM use and remains one of the more common ones.
The APP/DEP frequencies listed tell pilots that, if they desire to talk with ATC (as IFR flights probably would!), they need to contact Montreal Centre on 135.025 once airborne. Montreal Centre is also the arrival frequency that would help IFR arrivals get established on an instrument approach procedure, if one existed. Centre most probably does not have radio coverage to the ground on this frequency, and even if they did they would not provide towered-airport control services.

Its a mandatory frequency airport which means that aircraft are supposed to notify center when they arrive or depart.

Do you have a reference for that requirement? I couldn't find it in the CARs or the AIM.

I couldn't find that requirement either. In fact transport canada (section 4.5.7) just says you have to contact the MF/ATF on arrival and departure. It also says that if the ground station (UNICOM in this case) is not in operation, then all reports should be broadcast. There is not moention of contacting centre.

Your link says that arriving and departing must communicate through the published frequency their position and intentions. It does not require those communications to be with Center or even ATC. This only differs from other Class G airports by the requirement for communication. Normally, Class G without a control tower does not require a pilot to make radio calls. In the US, radio calls in uncontrolled airspace is only very strongly recommended and encouraged, but not required.

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text:
Why does operating the rudders in the engine slipstream offer improved rudder authority at low speed?
The above question is usually presented as an advantage for H-tail configurations. How does it help?

This is two questions @Zaber, please split them up.

A propeller accelerates the air flowing through it in order to create thrust. This acceleration is highest at low speed.
The acceleration will increase the dynamic pressure at the tail if it is in the slipstream. A higher dynamic pressure will cause a proportionally higher aerodynamic force. A H-tail will, therefore, only be in the slipstream if two engines are mounted ahead of it, like in two- or four-engined designs with wing-mounted propellers. Also, by placing the area of the vertical low, a H-tail offers better dynamic flying characteristics.


text:
What is the maximum wind speed a Boeing 737-800 can deal with during takeoff and landing?
I'd like to know how safe the Boeing 737-800 is during takeoff and landing when it's windy. What are the maximum wind speeds for cases of tail, head and cross winds?
EDIT
I've found this resource http://www.b737.org.uk/limitations.htm#Wind_Limits, but I'm quite unable to understand the data:

According to the table (which only shows x-wind data), the max wind speed depends on the so called "braking action", but I don't know what exactly is nor what it depends on.
EDIT 2
The same resource points out the max tailwind speed, as well as other info:

As your source shows, the maximum tail wind is fixed and fairly simple.
Getting slightly more complex, there is technically no maximum headwind I'm aware of, although most airports will close when the wind gets much above 50 knots, and even with the wind coming almost straight down the runway, you still get some crosswind component, which factors in. Expect to see many pilots divert in winds much above 40 knots even if the airport is open, especially if gusting to above 50.
Crosswinds are a little different as they depend on the braking action - basically how grippy the runway is. Simplified, you can think of it as a scale from dry weather, through slightly and very wet, then slush and ice. The grippier the runway, the more the tyres will grip and the higher crosswind the aircraft can deal with. The higher numbers in the chart you show are for a dry runway (good on your chart) the lower numbers for wet or icy (poor on your chart) runways.
Although it's written in terms of 'braking action', it's really a measure of grippiness, and it's important both when braking, but also for the ability of the tyres to prevent the aircraft sliding laterally off the runway. This job doesn't solely come down to the tyres, but they're part of it. This is also a large part of the reason why landings and takeoff have different maximum crosswinds on some aircraft.

If we are talking about 737 then there is maximum headwind. Landing speed for 737 is Vref + 1/2 headwind and cannot exceed Vref + 20. That means that maximum headwind component can not be more than 40knot

...source?

The posted information here is not correct.  The ADDITIVE doesn't go above +20 knots; that computation process doesn't drive a max headwind component.

@RalphJ, maybe i'm reading it wrong. Does it mean that you can't do landing with more than 40knots? Or it means that you can do more than 40 knots, but the additive component is limited to 20 knots?

The latter. The additive is limited to no more than +20, even if the computation would give a higher number. For instance, a headwind of 22 gusting to 32... the additive would compute out as 11 (half the steady value of 22) plus 10 (the gust increment) equals +21. But you'd only add +20. Don't believe Boeing publishes a "max headwind" limitation the way they publish the max tailwind operating limitation.